In a patent application entitled "Electronic Key Locking Circuitry," Ser. No. 163,281, filed 3/2/88 in the name of Lee, Robert D. et al., there is described an electronic key which provides access to a random access memory upon receipt of a valid password. Included within one embodiment of this electronic key is a timing circuit which provides access to the RAM for a limited time only. This timing circuit is calibrated by the manufacture prior to shipment, and the calibration must be protected so that it cannot be altered by someone other than the manufacturer except in a manner specified by the manufacturer.
Since the integrated circuit is packaged with a backup battery and since the calibration is dependent on the characteristics of the battery, then it is convenient to assemble the integrated circuit and battery as one module and then to perform the calibration after the module has been assembled. However if the calibration is to remain secure inside the integrated circuit, then a method must be used to lock out further calibration adjustments. In the application referenced above a fusing element is used which, when blown, prohibits further access to the calibration circuitry.
The circuit used in conjunction with the fuse element must thus be able to accommodate the currents and voltages required to "blow" the fuse, and further to detect whether the fuse has been blown. Furthermore, the fuse circuitry should be configured in such a manner that the security of the calibration cannot be breached by applying a specific voltage pattern to the pins of the integrated circuit in order to override the effect of the blown fuse.
Therefore, it can be appreciated that a diode and detection circuit which permits a fuse element to be blown, which detects the condition of the fuse element, and which is resistant to circumvention by an end user is highly desirable.